The present invention relates to a semiconductor integrated circuit such as a microcomputer and microprocessor and more specifically to a technique which may be effectively applied to protect secret information of chip from illegal attack by hackers, for example, the technique which may be effectively applied to a microcomputer and microprocessor as a data processing device to be built into an IC card.
In recent years, instead of magnetic card as an information storage medium, attention is paid to a microprocessor as a data processing device comprising a built-in central processing unit (CPU) and an IC card comprising a built-in semiconductor integrated circuit (IC) such as microprocessor. An IC card is thought as a medium which can be effectively applied into a cash-card or a credit-card because of its higher security in comparison with a magnetic card. However, such IC card is used to process the data having higher secrecy of information because of its higher security. Therefore, it is probable that an ill-intentioned third party such as a hacker considers this IC card as an object for misappropriation of secret information with disassembling and analysis thereof in view of producing a forged IC card.
As a means for illegal attack to a microcomputer with a hacker, it is said to introduce a means to realize erroneous operation of a microcomputer by increasing the clock frequency. In order to protect secret information of a microcomputer for IC card from illegal attack by a hacker, there is known the technique that a circuit is provided to monitor the clock frequency within the microcomputer for IC card and a reset signal is generated to disable operation thereof when the clock higher than the predetermined frequency is inputted thereto.
A frequency detecting circuit which detects whether the clock frequency is within the specification or not is described in the Japanese Laid-Open Patent Publication No. Hei 10(1998)-288635.